Transparent electrically-conductive layers of metal oxides such as indium tin oxide (ITO), antimony doped tin oxide, and cadmium stannate (cadmium tin oxide) are commonly used as transparent electrodes in electro-optical devices such as liquid crystal displays. Recently, such transparent electrically-conductive have also been considered for use in switchable mirrors having variable reflectivity to adapt to varying lighting conditions.
A switchable mirror has the ability to change reflectivity in response to an applied electric field. Such a mirror is useful as a rear view mirror for a vehicle, as a vehicle operator may darken the mirror to reduce glare from lights of a following vehicle.
Such a mirror may be constructed, for example, by sandwiching a liquid crystal material between two glass sheets, forming, in effect, a liquid cell. One of the glass sheets is coated one side with a metal layer, such as silver or aluminum, to form a reflector. The other sheet is coated on one side with a transparent electrically-conductive layer. The sheets are sandwiched together with the coated sides toward the liquid crystal material. The metal layer and the transparent electrically-conductive layer form electrodes for applying an electric field to the liquid crystal material. The transparent electrically-conductive layer may be referred to as a transparent electrode. The metal layer may be referred to as a reflective electrode.
The above-described switchable mirror is normally used with the transparent electrode towards the vehicle operator. Because of this, the highly reflective mirror is viewed through the transparent electrode and the liquid crystal.
A problem that has been encountered in the design of switchable mirrors is that maximum brightness is limited by visible-light absorption in glass, transparent electrode, and the liquid crystal. Maximum brightness is also limited by reflection losses at optical interfaces between the glass, transparent electrode, and the liquid crystal. This causes the highest available reflectivity of such a mirror to be significantly less than a conventional rear-view mirror.
By way of example, a conventional rear view mirror has a reflectivity between about eighty and ninety percent. It has been found difficult, however, to construct switchable mirrors which have a maximum reflectivity greater than about seventy percent. Seventy percent is a generally accepted minimum standard reflectivity for vehicle rearview mirrors.
A liquid crystal display device also includes a liquid cell arrangement in which a liquid crystal material is sandwiched between two electrode-coated sheets. In a reflective display, the cell includes reflective and transparent electrodes as described above for the switchable mirror. In a transparent or back-lit display, a transparent electrode takes the place of the reflective electrode, i.e., both electrodes are transparent. In a liquid crystal display device reflection from electrode glass and liquid interfaces often contribute to reducing contrast and brightness of the display.
Absorption losses in materials of a switchable mirror or a display device may be minimized by appropriate material selection and processing. At best absorption losses may be reduced to an intrinsic level characteristic of the materials. Reflection occurs as a result of reflection caused by a refractive index (n) mismatch between the metal oxide material of the transparent electrode, the glass on which it is coated, and the liquid crystal material. Such reflection may range from a low of about two percent (for an electrode layer having an optical thickness of about one-half wavelength of visible light) to greater than ten percent (for an electrode layer having an optical thickness of two wavelengths or greater of visible light, or for an electrode layer having a thickness of one-quarter wavelength of visible light or less).
At first consideration, it may appear that, device cost permitting, losses in a transparent electrode system may be reduced by incorporating a transparent conductive layer as one layer in an optical interference layer system which forms, in effect, an antireflection coating between a glass substrate on which the layer system is deposited and the liquid crystal material. While apparently simple in concept there are particular aspects of the design and construction of liquid cells and liquid cell display devices which have limited the extent to which interference layer methods have been used to reduce reflection problems in transparent electrodes.
One problem is presented when a transparent electrode is required to have a very low sheet resistance, for example, on the order of one ohm per square (1 .OMEGA.n/sq.) or less. Such a low sheet resistance may be required, for example, to reduce electrical losses in the layer or to allow the electrode to operate with a low voltage power supply, such as a battery. Low sheet resistance is often accomplished by providing a layer of a transparent conductive material which may have a thickness two to three wavelengths of visible light. ITO and fluorine doped tin oxide, both having a refractive index of about 2.0 for visible light, are believed to the most commonly used transparent conductive materials.
In the design of conventional antireflection coatings, it is a problem to accommodate a thick layer of a material having a refractive index of about 2.0 in the layer system of the coatings. This problem, and methods for at least partially solving the problem, are taught by Dickey in U.S. Pat. No. 5,105,310 and Austin in U.S. Pat. No. 5,147,125. The methods taught however appear to only be applicable if layers having a refractive index of about 2.0 have a thickness of one wavelength or less for visible light.
Transparent electrodes in display devices are typically disposed on a glass substrate in the form of a pattern. The pattern corresponds to numbers, characters, or designs, in the display. Forming electrodes in such patterns is usually accomplished by a coating a glass sheet with a continuous layer of a transparent electrically-conductive material and then etching away the electrically-conductive material where it is not required. Un-etched areas form the electrodes in the pattern and the electrodes are electrically isolated from each other by the etched areas. The etched and un-etched areas typically have a different reflectivity, and, more often than not, a different reflection color. Because of this, the electrode pattern is visible even when the device is not activated.
The reflection at a liquid glass interface is relatively low. Typically it may have a value between about 0.1 and 0.2 percent. The electrode visibility problem, however, may not be solved simply by reducing the reflection of the electrode to a low level as possible, as differences in small reflection values may still be detectable if the reflection (contrast) ratio between electrode and intervening areas is high, or if there is a perceptible color difference between the reflection colors in the electrode and intervening areas.
There is clearly a need to improvement optical properties of transparent electrodes for electro-optical liquid cell devices such as switchable mirrors and display devices.